With the increase in functions, increase in complexity, and systemization of electronic devices represented by a digital television, methods and procedures for their use have become increasingly complex in recent years. In addition, with the diversification of a user's use situation and usage pattern, a Human-Machine Interface (HMI) that has been customized to suit the state of a person is in demand, and a device or method for accurately estimating and measuring a human state such as the psychological state, the emotional state, and the thinking state of the user is becoming a very important element.
Conventionally, there have been various attempts at estimating such human states (for example, see Patent Reference 1). Patent Reference 1 proposes a fixational eye movement checking device which detects the movement of an eyeball of a subject when indices are presented on an index board, and distinguishes an abnormality in the fixational eye movement component from the time variation in the line of sight movement based on the detected eyeball movement, so as to determine a brain function-related disorder. This device checks for abnormalities in the eyeball movement control mechanism in the brain of the subject by measuring and analyzing the fixational eye movement of an eyeball of the subject, and thereby facilitates the distinguishing of cerebrovascular dementia, and so on.
It should be noted that “fixational eye movement” is one type of eyeball movement, and is the fine trembling of the eye which occurs involuntarily at all times even when staring at a still object.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 06-154167